The concept of gas-depolarized galvanic or electrochemical cells is well known. In such cells, there is an anode and a gas-depolarizable cathode, usually having an electrolyte carrying separator disposed between the anode and cathode, and having some means whereby there is gas communication between the gas-depolarizable cathode and the ambient. Such means include gas communication passageways through the container of the cell. Frequently, the depolarizing gas is oxygen from the ambient air; and most often the metal used in metal-air cells is zinc, but cadmium, iron or other metals may also be used.
In any event, metal-air cells have a higher energy density than the usual "dry" cells or other primary cells such as zinc/manganese dioxide or zinc/mercuric oxide cells; and in the most common instance, zinc-air cells may have three times or more the energy density of such other cells.
Likewise, there are a number of applications for gas-depolarizable galvanic cells such as zinc-air cells where it is desired to operate the cell in such a manner as to draw current from it at very low steady current rates, with occasional very short high current requirements. Some such applications may include hearing aids and the like, but particularly such applications for zinc-air cells according to the present invention are contemplated as electronic watches, calculators and the like. Electronic watches may include analog watches which generally have a steady current draw but which may require an intermittent higher current to drive a wheel or disc upon which a day/date display is shown; and more particularly, electronic watches include LED (light emitting diode) or LCD (liquid crystal display) watches, which have very low current draw during normal operation to drive the watch module, and as well for LED watches or calculators to power the constant liquid crystal display -- such current draws being only in the order of microamperes -- but where occasional high current rate for a very short period of time may be required such as to illuminate the LED display or to power a back light for an LCD display. However, for tritium oxide LCD backlight, such higher current pulses are not required.
In general, it is desirable to restrict the gas flow communication to the gas-depolarizable cathode of the cell -- the air or oxygen cathode -- thereby restricting water vapour and oxygen access or egress from the cell, and in some instances to control the limiting current which may be drawn from the cell at its rated terminal voltage.
It is particularly to be noted that a major consideration is to assure that there is a long operating life for a zinc-air cell or the like in such low rate applications with occasional high rate, short term current draw as discussed above; and that such assurance of long operating life may be particularly attained by precluding either moisture loss from the alkaline electrolyte of the cell in low relative humidity ambient conditions, or excessive water vapour absorption by the alkaline electrolyte of the cell at high relative humidity ambient conditions. Obviously, if the alkaline electrolyte is permitted to lose its moisture content and therefore to dry out, the cell may self-discharge or otherwise become inoperable because of internal short circuits or other damage; whereas, if there is too much water vapour influx and absorption by the electrolyte, the physical dimensions of the cell may be altered in that it may be caused to bulge and thereby to leak electrolyte from the cell or again to cause other damage -- and in any event, in such applications as electronic watches a physical dimensional change of the cells installed in the watch cannot usually be accommodated except within very narrow limit of tolerance.
In all events, there is also the possibility that where there is substantially unrestricted air access to the interior of the cell, there may be some carbon dioxide absorption by the alkaline electrolyte, thereby decreasing the energy capacity and power output capability of the cell.
Thus, the present invention provides for the physical interposition of a non-porous barrier in the path of gas communication to the cathode of the cell -- where the barrier may be the cell container or it may be an additional element placed within the cell -- and where the gas passageway through the barrier has a restricted area such as to restrict gas flow rate to the cathode; and because, in general, the present invention is particularly adapted to button cells for use in electronic watches and the like, this invention provides that the total cross-sectional area of the gas passageway through the barrier perpendicular to the direction of gas flow therethrough is not greater than the total projected area of the gas passageway through the thickness of the barrier. It will be recognized that, in general, it is not possible to punch, pierce or drill holes, or to form slits, through non-porous material -- especially sheet metal or metal foil -- where the width or diameter of the aperture or slit is less than the thickness of the thin material. Where a very thin foil or plastics material may be used, it may be possible to form slits or to pierce or punch apertures therethrough having a diameter or width which is approximately equal to the thickness of the material; but where metal sheet, for example, is used as in cell containers, it is not possible to place a very restricted aperture through the metal sheeting using ordinary manufacturing techniques. Accordingly, it is a feature of the present invention that, especially where an aperture is formed in the cell container, that aperture may be formed by exposing the portion of the cell container or the metal from which it is to be made where the aperture is to be formed to a laser beam or to a chemical etching process, whereby a very small but well controlled aperture may be formed. The manufacturing process may also include forming apertures in sheet metal by laser drilling or chemical etching techniques; and thereafter forming the cathode container by punching and drawing a blank from the metal sheeting where an aperture has been made.
Therefore, it is possible by this invention to provide low rate, special purpose metal-air alkaline electrolyte cells using ordinary components which might otherwise be used for high rate cells, merely by altering or adding one or more additional manufacturing steps during the assembly of such a cell to obtain the desired low rate cell having restricted gas flow communication to the cathode thereof.
What the present invention provides, therefore, is means whereby special purpose cells may be provided using ordinary cell components with the simple provision of a very restricted gas passageway through a barrier which is interposed in the path of gas communication from the ambient of the cell to the cathode thereof. Excessive moisture vapour influx or egress to or from the cell, respectively, depending upon the relative humidity of the ambient in which the cell is operating, is thereby assured.
There is, of course, some accumulation of oxygen within the interior of the cell during normal low rate operation thereof, particularly within the air diffusion layer or region of the cell, such that there is sufficient excess depolarizing gas within the cell to permit short term, high current rate draw therefrom.
It should be noted, of course, that the present invention is not restricted to button cells, and may be applicable to larger cylindrical cells. In such circumstances, however, there may be a plurality of very fine gas passageways formed through the cell container and/or a separate barrier placed within the cell, within the teachings of the present invention, and thereby providing controlled gas access to the cathode of the cell. Likewise, it is clear that the present invention is not restricted to zinc-air cells, but such cells are the most common gas-depolarizable galvanic cell which would be found in low rate applications of the sort discussed above.